PHOTOCATALYSIS OF OLIGO(PARA-PHENYLENES) - PHOTOREDUCTIVE PRODUCTION OF HYDROGEN AND ETHANOL IN AQUEOUS TRIETHYLAMINE

Oligo(p-phenylenes) (OPP-n), p-terphenyl (OPP-3) to p-sexiphenyl (OPP-6), catalyze water-reductive H2 formation and reduction of concomitantly formed acetaldehyde to ethanol upon irradiation of heterogeneous suspensions in aqueous organic solution in the presence of triethylamine (TEA) and RuCl3. Colloidal Ru0 is photoformed in situ to work as an electron relay. The activity of OPP-n increases with the number of phenylene units except for the cases of OPP-3 and of the alkylated derivatives, where the net photocatalytic activities are higher, mainly due to the effective homogeneous catalysis, since their solubilities in the solvents employed are significantly larger. The homogeneous photocatalysis of OPP-3 leads not only to H2 evolution but also to effective formation of ethanol in the absence of colloidal Ru0, being accompanied by photo-Birch reduction of OPP-3. Dynamics studies of OPP-3 reveal that photocatalysis should be initiated by formation of the excited singlet state of OPP-3 (1OPP-3*), which is reductively quenched by TEA at a rate controlled by diffusion to produce the OPP-3 radical anion (OPP-3.-) and the TEA radical cation (TEA.+). From laser flash photolysis and pulse radiolysis experiments, it is concluded that electron transfer from OPP-3.- leads to effective reduction of water to H2 catalyzed by Ru0 colloid. Furthermore, it is confirmed that OPP-3.- gives electrons directly to acetaldehyde without any electron relays like colloidal metals, resulting in the formation of ethanol. During photocatalysis, OPP-3 itself undergoes photo-Birch reduction to some extent.